diff --git a/.travis.yml b/.travis.yml
index fdc45dd..d213dc9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,11 +1,13 @@
+dist: trusty
+
 language: php
 
 php:
-  - 5.5
   - 5.6
   - 7.0
   - 7.1
-  - nightly
+  - 7.2
+  - 7.3
 
 before_script:
   - wget http://getcomposer.org/composer.phar
diff --git a/composer.json b/composer.json
index 8e72f1b..1aef8a6 100644
--- a/composer.json
+++ b/composer.json
@@ -12,12 +12,12 @@
         }
     ],
     "require": {
-        "php": ">=5.5.0",
+        "php": ">=5.6.0",
         "guzzlehttp/guzzle": "^5.0|^6.0",
-        "symfony/serializer": "^2.0|^3.0"
+        "symfony/serializer": "^2.0|^3.0|^4.0|^5.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.0"
+        "phpunit/phpunit": "^5.0"
     },
     "autoload": {
         "psr-0": {
diff --git a/tests/Josser/Tests/ClientTest.php b/tests/Josser/Tests/ClientTest.php
index 322f582..cf1abd4 100644
--- a/tests/Josser/Tests/ClientTest.php
+++ b/tests/Josser/Tests/ClientTest.php
@@ -29,17 +29,17 @@ class ClientTest extends JosserTestCase
     public function testRequest($requestMethod, $requestParameters, $responseResult)
     {
         /* @var $transport \Josser\Client\Transport\TransportInterface */
-        $transport = $this->getMock('Josser\Client\Transport\TransportInterface');
+        $transport = $this->getMockBuilder(Client\Transport\TransportInterface::class)->getMockForAbstractClass();
         /* @var $protocol \Josser\Protocol\Protocol */
-        $protocol  = $this->getMock('Josser\Client\Protocol\Protocol');
+        $protocol  = $this->getMockBuilder(Client\Protocol\Protocol::class)->getMockForAbstractClass();
         /* @var $response \Josser\Client\Response\ResponseInterface */
-        $response = $this->getMock('Josser\Client\Response\ResponseInterface');
+        $response = $this->getMockBuilder(Client\Response\ResponseInterface::class)->getMockForAbstractClass();
         $response->expects($this->any())
                  ->method('getResult')
                  ->will($this->returnValue($responseResult));
 
         /* @var $client \Josser\Client */
-        $client = $this->getMock('Josser\Client', array('call'), array($transport, $protocol));
+        $client = $this->getMockBuilder(Client::class)->setConstructorArgs([$transport, $protocol])->setMethods(['call'])->getMock();
         $client->expects($this->once())
                ->method('call')
                ->with(
@@ -64,17 +64,17 @@ public function testRequest($requestMethod, $requestParameters, $responseResult)
     public function testNotify($requestMethod, $requestParameters, $responseResult)
     {
         /* @var $transport \Josser\Client\Transport\TransportInterface */
-        $transport = $this->getMock('Josser\Client\Transport\TransportInterface');
+        $transport = $this->getMockBuilder(Client\Transport\TransportInterface::class)->getMockForAbstractClass();
         /* @var $protocol \Josser\Protocol\Protocol */
-        $protocol  = $this->getMock('Josser\Client\Protocol\Protocol');
+        $protocol  = $this->getMockBuilder(Client\Protocol\Protocol::class)->getMockForAbstractClass();
         /* @var $response \Josser\Client\Response\ResponseInterface */
-        $response = $this->getMock('Josser\Client\Response\ResponseInterface');
+        $response = $this->getMockBuilder(Client\Response\ResponseInterface::class)->getMockForAbstractClass();
         $response->expects($this->any())
                  ->method('getResult')
                  ->will($this->returnValue($responseResult));
 
         /* @var $client \Josser\Client */
-        $client = $this->getMock('Josser\Client', array('call'), array($transport, $protocol));
+        $client = $this->getMockBuilder(Client::class)->setConstructorArgs([$transport, $protocol])->setMethods(['call'])->getMock();
         $client->expects($this->once())
                ->method('call')
                ->with(
@@ -103,4 +103,4 @@ public function requestAndNotificationDataProvider()
             array('divide', array(3, 2), 1.5),
         );
     }
-}
\ No newline at end of file
+}
diff --git a/tests/Josser/Tests/Protocol/JsonRpc1Test.php b/tests/Josser/Tests/Protocol/JsonRpc1Test.php
index b968244..bc94dac 100644
--- a/tests/Josser/Tests/Protocol/JsonRpc1Test.php
+++ b/tests/Josser/Tests/Protocol/JsonRpc1Test.php
@@ -290,11 +290,11 @@ public function validRequestsDataProvider()
      */
     public function invalidRequestsDataProvider()
     {
-        $mock = $this->getMock('Josser\Client\Request\RequestInterface');
-        $mock->expects($this->atLeastOnce())
+        $mock = $this->getMockBuilder(RequestInterface::class)->getMockForAbstractClass();
+        $mock->expects($this->any())
              ->method('getMethod')
              ->will($this->returnValue('mocked.math.sum'));
-        $mock->expects($this->atLeastOnce())
+        $mock->expects($this->any())
              ->method('getId')
              ->will($this->returnValue(123324234));
 
@@ -364,4 +364,4 @@ public function requestsAndNotificationsDataProvider()
             array(new Notification('system.exit', array()), true),
         );
     }
-}
\ No newline at end of file
+}
diff --git a/tests/Josser/Tests/Protocol/JsonRpc2Test.php b/tests/Josser/Tests/Protocol/JsonRpc2Test.php
index bec3939..f424500 100644
--- a/tests/Josser/Tests/Protocol/JsonRpc2Test.php
+++ b/tests/Josser/Tests/Protocol/JsonRpc2Test.php
@@ -311,11 +311,11 @@ public function validRequestsDataProvider()
      */
     public function invalidRequestsDataProvider()
     {
-        $mock = $this->getMock('Josser\Client\Request\RequestInterface');
-        $mock->expects($this->atLeastOnce())
+        $mock = $this->getMockBuilder(RequestInterface::class)->getMockForAbstractClass();
+        $mock->expects($this->any())
              ->method('getMethod')
              ->will($this->returnValue('mocked.math.sum'));
-        $mock->expects($this->atLeastOnce())
+        $mock->expects($this->any())
              ->method('getId')
              ->will($this->returnValue(123324234));